Engine designers are continually seeking improvements in engine design which improve engine efficiency. One manner of improving engine efficiency is to improve the operational efficiency of the fuel system. Specifically, any leakage of high pressure fuel within the fuel system represents wasted energy that can reduce engine efficiency. Loss of high pressure fuel has recently become an even greater problem as injection pressure levels are increased in an effort to improve fuel economy and reduce emissions as required by recent and upcoming legislation.
Undesirable leakage of fuel often occurs in a component of the fuel system having a member, such as a valve element or a fuel plunger, reciprocally mounted in a bore formed in a body and sized to form a close sliding fit with the inside surface of the body to create a partial fluid seal between the adjacent surfaces. As the fuel pressure increases, a pressure gradient is developed along the length of the seal, i.e., clearance, between the member and opposing wall forming the bore. The extent of the leakage flow through the clearance depends primarily on the magnitude of the pressure gradient, the engagement length, the size of the operating clearance and the fluid viscosity. The size of the operating clearance is affected by the amount of fuel pressure induced dilation or deformation of the body forming the bore. One manner of reducing the leakage is to design the components to achieve a smaller clearance between the plunger and barrel. However, the practice of requiring closer tolerances increases manufacturing costs. Another method of reducing leakage is to design the body to resist pressure induced dilations by increasing the size and/or strength of the body or housing forming the bore. However, this method undesirably increases the size and weight of the components and, thus, the fuel system.
Many fuel systems used in contemporary engines include a reciprocally mounted fuel pressurization plunger incorporated into, for example, a unit fuel injector, such as disclosed in U.S. Pat. No. 5,072,709, or a fuel pump assembly, such as disclosed in U.S. Pat. No. 4,530,335. Each plunger is typically either mechanically or hydraulically operated to pressurize fuel in a pressure chamber for injection into the engine cylinder. For example, U.S. Pat. No. 5,096,121 and 5,441,027 disclose hydraulically actuated intensification plunger assemblies. However, these references do not suggest reducing the leakage between the plunger and adjacent bore wall and, therefore, are subject to the disadvantages discussed hereinabove.
U.S. Pat. No. 4,991,495 to Loegel, Sr. et al. discloses a pumping mechanism including a plunger mounted in a bore and a plurality of inserts positioned in series along the plunger for sealing the space between the plunger and its housing. The inserts include thrust and sealing rings which deform and expand radially in response to axial fluid-induced forces imparted by adjacent inserts. However, this sealing assembly requires an excessive number of discs and other parts creating a complex and expensive arrangement which is costly and difficult to maintain. Moreover, this device undesirably requires the resealing of two gaps, one on each side of the inserts, during each pumping stroke of the plunger thus increasing the likelihood of less than minimal leakage.
U.S. Pat. No. 5,038,826 to Kabai et al. discloses a three-way valve including a piston slidably positioned in a valve body. High pressure fuel is delivered to the valve via aligned ports formed in the valve body and the piston. An integral portion of the piston or the valve body is acted upon by supply fuel pressure to reduce the clearance between the piston and a valve body thereby reducing the leakage between the components. Although deformation of the integral portion tends to close the clearance gap to reduce leakage, the resulting close tolerances may result in increased wear, or possibly scuffing, of the valve body or piston resulting, over time, in excessive clearances. For the Kabai et al. design, excessive wear would eventually require replacement of the entire piston and/or valve body, unnecessarily increasing costs. Also, the integral portion disadvantageously provides reduction in the pressure gradient over only a limited, localized portion of the seal length and thus fails to minimize leakage in an optimum manner. In addition, the integral portion is formed by machining internal passages into the valve body or piston undesirably increasing manufacturing time and costs.
Consequently, there is a need for an improved plunger and barrel assembly which effectively and optimally minimizes fluid leakage through the clearance between the plunger and barrel while minimizing the costs and size of the assembly.